The invention relates to vacuum chambers and more specifically the joining together of casing elements used in vacuum chambers. The invention is particularly useful in a particle accelerator in which a high vacuum is realized. The invention is not limited to this use and can be implemented in any type of vacuum chamber. They are used for example for high-power lasers and for space simulators.
In a particle accelerator, it is common to carry out helium leakage tests in order to achieve values of around 10−10 Pa·m3/s. In addition to such leaktightness, the chamber is made of materials that outgas as little as possible in order to avoid any disruption of the experiments carried out in the accelerator.
Vacuum chambers cannot be made of one-piece mechanical parts and assemblies are necessary. These assemblies have to be disassemblable so as to allow operators to work inside the chambers and to allow the connection of equipment such as vacuum pumps or measurement sensors.
One known solution for producing an assembly that is usable in a vacuum chamber consists in flanging two mechanical elements to be joined together. More specifically, the elements to be joined together have complementary faces, between which a seal is pressed. In order to avoid the possible outgassing of the seal, it is preferable to use a metal seal, the rate of outgassing of which is around one hundred times lower than that of elastomeric seals.
In order that the surfaces between which the seals are pressed are not damaged, the hardness of the seals has to be lower than that of the facing surfaces. It is possible, for example, to produce the casing elements of the chambers from stainless steel, in particular steels known under their standard designation of type 304L according to the American standard AISI or 1.4306 according to the European standard EN 10027, or of type 316L (American standard) or 1.4404 (European standard). These steels are known notably for their excellent weldability, this making it easier to assemble the chambers. Conventional seals fitted between surfaces made of such steels are made of copper alloy or aluminum alloy.
It may be necessary, notably on account of weight constraints, to make the casing elements of the chambers from more lightweight metals, notably from aluminum alloy. Some aluminum alloys moreover have many advantages for producing vacuum chambers, such as their low rate of outgassing, their cost, their ease of machining, their magnetic permeability, their low activation by neutrons, much lower hydrogen permeation than stainless steel, their thermal conductivity making it possible to more easily stove the vacuum chambers and to dissipate heat inputs without additional cooling, etc.
Some alloys containing zinc should be avoided because their vapor pressure is not compatible with the ultrahigh vacuum necessary in a particle accelerator. Although the weldability of aluminum alloys is generally less good than that of stainless steels, some alloys from the standard series 1000, 5000 and 6000 according to the European standard EN 485-2 have very good weldabilities.
In order to allow the fitting of metal seals between assembly surfaces, one American company: Atlas Technologies, the headquarters of which are at 3100 Copper Avenue in Fenton in the state of Michigan, has developed aluminum flanges covered with a surface of stainless steel. The aluminum part of the flange can be welded to a casing element of a vacuum chamber and the stainless steel surface makes it possible to squeeze a metal seal, for example a copper seal. The flanges of this company are widely used in renowned particle accelerators, notably that of CERN located close to Geneva, Switzerland.
The Applicant company has encountered difficulties in employing these flanges, resulting in a reject rate of around 20%, notably on account of the occurrence of leaks at the stainless steel/aluminum junction. These flanges are fitted on the casing elements of the vacuum chambers by fusion welding, this also involving risks of porosity in the welds. Another obstacle to the use of these flanges is their high cost.